1. Field
This invention relates generally to charge pump voltage regulators, and more specifically a charge pump voltage regulator for use under low input voltage conditions.
2. Related Art
Most charge pumps are implemented with high voltage transistors. In this context, high voltage transistors are transistors that can support a drain-to-source voltage and a gate voltage that are relatively higher than the drain-to-source and gate voltages that medium and low voltage transistors can support. The threshold voltage VT of high voltage transistors is typically 700-800 mV and it could be sometimes higher. Many charge pumps do not work properly at low input voltage VIN conditions (e.g., when VIN is above the threshold voltage VT of high voltage transistors by 150-200 mV or less) and at a proper clock frequency for the charge pump. Therefore, many charge pumps do not work properly when VIN is less than about 1V.
A charge pump circuit comprises a charge pump and a regulator. Most charge pump circuits do not provide adequate line and load regulation when the input voltage goes below a nominal VIN because known charge pumps have their line and load regulation limited by V. Therefore, in low VIN applications, such as when VIN is provided by a low voltage battery, known regulators for charge pumps cannot always maintain a constant output voltage. For example, under certain low VIN conditions, known charge pump circuits may have difficulty maintaining the output voltage at a target voltage when the battery becomes weak. When VIN is about 1V or lower, and the target output voltage is higher than VIN, known regulators may fail to maintain the target output voltage of known charge pumps.
Most charge pump circuits include a clock generator circuit for generating clock signals that are coupled to capacitors in the charge pump. The output voltage of most charge pumps is regulated by modulating the frequency of such clock signals.
In known regulators for known charge pumps, the clock generator circuit is powered by VIN. Therefore, a maximum amplitude of clock signals that can be produced by the clock generator circuit of such known regulators is VIN. Consequently, in battery-powered applications, the maximum amplitude of the clock signals that can be produced by the clock generator circuit is a voltage of a battery. A maximum output voltage of such known charge pumps is limited by the highest amplitude of the clock signals produced by the clock generator circuit. Therefore, in battery-powered applications, the maximum output voltage of such known charge pumps is dependent on, and limited by, the voltage of the battery. The output voltage VOUT of a charge pump is also dependent, in part, on a number of stages in the charge pump, an amount of current drain on the charge pump, capacitance of an output capacitor, and a frequency of the clock signals coupled to the capacitors in the charge pump.